When a pneumatic cylinder is used as a power source for a machine tool device, many times the cylinder is controlled by a neutral-centered valve. In case of a failure, either in the valve or the cylinder or the controls thereof, the trapped air remains on both sides of the piston. In the event of an unsafe condition, the cylinder may be stopped by allowing the neutral-centered control valve to go to a center position, trapping air on opposite sides of the piston, and stopping the machine tool device in this position.
Should a maintenance person attempt to service the equipment, even if the air is shut off, trapped air will remain on both sides of the piston. Should he, by manually operating the valve, dump the air from either side of the piston, the machine tool device will then move under the expanding air on the opposite side of the piston and may cause injury to himself or other personnel.